It's actually pretty easy to convert themes now a days with PatPat's CTFTool GUI (The link is in my sig) Once you install the tool just right click on a theme and find the firmware you want the theme to be and click that, wait for the tool to finish converting and you'll have the converted theme right next to the old theme (with the 6.xx version you converted to at the end of the theme).
As for PRO-B9 you can get the official PRO updater in the "Custom Firmwares" forum (in this thread
@ your edit : Delete all the themes from your THEME folder in your PSP folder and put themes you know are for 6.35 in there. Also go into the CXMB folder and click on the config.txt file and delete all the text inside, then save and apply a new theme.
P.S. I suggest downgrading to 6.20 (since you're on a 3g you can do that) because then you can use any of the themes that I convert (I use 6.20PRO-B9)